Method and apparatus for pre-heating recirculated flue gas to a dry scrubber during periods of low temperature

Abstract

A method and system for preventing exclusions in an air quality control system (AQCS) of a type having a scrubber, either a circulating fluidized bed (CFB), transport reactor (TR) or spray-dryer absorber SDA, and in all three cases with a flue gas recirculation FGR system. The invention automatically pre-heats flue gas recirculated to the input of the CFB / TR / SDA scrubber during periods of low temperature operation. The system supplements the “traditional” gas recirculation scheme with heater(s) that are used during periods of low temperature. During periods of low load conditions, the system monitors the SDA / TR / CFB inlet temperature and, when it falls below a setpoint, the control system increases the heat input of the pre-heater(s) in the FGR system in order to maintain a minimum inlet temperature. This satisfies the need for the inlet temperature to be above a certain level, thereby ensuring that the temperature is high enough so that when process water is added for promotion of the acid gas reactions with lime the scrubber exit temperature is maintained above the minimum required for proper system operation. The temperature sensor may be a conventional dry bulb temperature sensor or its functional equivalent installed at or near the CFB, TR or SDA scrubber FGR inlet.

Description

CROSS-REFERENCE TO RELATED APPLICATION(S)[0001]The present application derives priority from U.S. Provisional Patent Application No. 61 / 704,073 filed 21 Sep. 2012.BACKGROUND OF THE INVENTION[0002]1. Field of the Invention[0003]The present invention relates to a method and system for increasing the operational range and efficiency of an Air Quality Control Systems (AQCS) incorporating both a Circulating Fluid Bed Scrubber (CFB) with flue gas recirculation (FGR) system by automatically pre-heating flue gas recirculated to the input of the CFB dry scrubber during start-up, shut-down and other periods of low temperature operation. The invention is also effective for Spray Dry Absorbers (SDAs) and Transport Reactor (TR) type dry scrubbers.[0004]2. Description of the Background[0005]Power companies' efforts to reduce sulfur dioxide and other emissions have focused largely on the use of advanced emission control equipment and improving operating practices. A number of different Air Quality...

AI Technical Summary

Benefits of technology

The system effectively maintains scrubber performance and prevents acid gas emissions excursions by ensuring adequate inlet temperature and flow rates, allowing dry scrubbers to operate efficiently during all load conditions, including start-up and shut-down, without compromising environmental regulations.

Problems solved by technology

However, the flue gas coming from the upstream boiler is very hot.

However, flue gas flow rate can sometimes become a problem during start-up, shut-down and other low load conditions.

Below certain minimum operational levels, poor acid gas removal efficiency is a direct result of low load operation, and low load operation is inevitable during unit startups and shutdowns, or during power plant cycling.

With the advent of new air pollution control regulations that do not permit any periods of non-compliance, even during boiler start-up and shutdown, taking the AQC System offline at low load is no longer an option.

Any excursion of high acid gas emissions rates can result in the plant owner being in violation of laws punishable by fines and worse if the plant is a chronic offender.

If the flue gas temperature entering the SDA, TR or CFB is too low, insufficient water or lime slurry injection occurs, resulting in poor acid gas removal efficiency.

Use of traditional FGR makes this issue worse as the cleaned flue gas used for gas recirculation is much cooler than the gases entering the scrubber system from the boiler or other source and, by itself, is much cooler than is allowable for proper scrubber operation.

Even with an FGR system it is possible to experience “excursion” issues, e.g., acid gas emissions leaving the scrubber system at values much higher than the allowable levels.

Thus, for any traditional FGR system it is not possible for a dry scrubber of any type to receive flue gas from the boiler or other source and to scrub it as required without an excursion unless the scrubber has first been brought up to minimum flow and temperature, and lime and water have been added to the system.

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